Update countries-where-sms-authentication-is-supported.md

[wyywwi]

Added "regions" to the list description for clarity. Kept the front matter unchanged to avoid potential unintended impacts.

Why:

The list description previously lacked "regions", which could lead to confusion for some users. To improve clarity and accuracy, "regions" was added to the description.

Closes:

What's being changed (if available, include any code snippets, screenshots, or gifs):

The term "country" was updated to "country or region" in the section titled Supported countries for SMS authentication. The title was also updated. The front matter was left unchanged to prevent potential unintended impacts on indexing and browsing.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require a SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing.

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Content directory changes

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Source	Preview	Production	What Changed
authentication/securing-your-account-with-two-factor-authentication-2fa/countries-where-sms-authentication-is-supported.md	fpt ghec	fpt ghec

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server

[nguyenalex836]

@wyywwi Thanks so much for opening a PR! I'll get this triaged for review ✨